Previously, it has been a common technique to utilize metals to extract polypeptides (e.g., proteins or protein fragments) from compositions. Such extraction has been based on the ability of a metal to complex with a specific polypeptide over other substances within the composition. It is known that two different peptide-based metal binding tags can be used for separating a peptide from a composition. However, these binding tags have specific types of amino acids, and specific numbers or specific sequences of amino acids that are utilized to accomplish binding that are substantially different from those described in connection with the present invention. For example, these two binding tags may not bind the same types of metals or bind using the same geometry as the present invention. Additional information regarding the first binding tag can be obtained in U.S. Pat. No. 7,208,138 B2 by Haroon et al., and pertains to a peptide comprising the sequence NXEQVSP. Information regarding the second binding tag can be obtained in U.S. patent application publication 20040018974 by Arbogast, et al., and pertains to a sequence of the tag that appears to be the entire protein described therein.
Tags that can be encoded in the genetic material of an organism for recombinant expression of proteins have been utilized extensively for purification and identification of protein products. The most noteworthy example of this has been the His-tag technology, which provides a facile means of effectively isolating the tagged protein from whole cells using immobilized metal affinity chromatography (IMAC). Numerous other peptide-based tags have been developed for detecting a tagged protein in cell culture assays or cell lysates using antibodies that recognize the peptide tag. These technologies are useful in in situ or in vitro assays, but they generally are not applicable to in vivo analysis. The advantage of a peptide tag is that the tag is covalently attached to the protein of interest without the need for additional chemical steps to label the protein.
MRI imaging is a common method for examining structural features in live animals and humans. The technology is safe and non-invasive. Contrast agents have been developed to improve the sensitivity of the method, and they are used to enhance the features observed in the MRI image. The improvement is achieved because contrast agents contain metals that alter the signal from neighboring molecules, typically water. The most commonly used agents chelate Gd(III), but other metals can be used to enhance contrast. Gd-containing contrast agents cannot be used in patients with compromised kidney function, as serious complications known as Nephrogenic Systemic Fibrosis or Nephrogenic Fibrosing Dermopathy can result. Alternative imaging agents are needed to address the needs of such patients. Compounds that chelate metals are also used in PET and SPECT imaging to view molecular level details.